Conventionally pneumatic rubber vehicle tires are produced by molding and curing a green or uncured and unshaped tire in a molding press. The green tire is pressed outwardly against a mold surface (sometimes called a negative mold surface) by means of an inner fluid-expandable bladder. By this method the green tire is shaped against the outer mold surface which defines the tire tread pattern and configuration of the side-walls. By application of heat and pressure the tire is molded and cured at elevated temperatures.
In general practice, the expansion of the bladder is accomplished by application of internal pressure to the inner bladder cavity which is provided by a fluid such as gas, hot water and/or steam which also participates in the transfer of heat for the curing or vulcanization of the tire. The tire after molding and curing is allowed to cool somewhat in the mold, some times aided by adding cold or cooler water to the bladder. Then the mold is opened, the bladder is collapsed by removal of its internal fluid pressure and the tire is removed from the tire mold. Such use of tire curing bladders is well known to those having skill in the art.
It is recognized that there is substantial relative movement between the outer contacting surface of the bladder and the inner surface of the tire during the expansion phase of the bladder. Likewise, there is considerable relative movement between the outer contacting surface of the bladder and the cured inner surface of tire during the collapse and the stripping of the bladder from the tire after the tire has been molded and vulcanized.
The bladder surface can tend to stick to a tire inner surface after the tire is cured and during the bladder collapsing part of the tire cure cycle. This adhesion may cause roughening of the bladder surface if it is not controlled. This reduces bladder durability and can produce defective tires. For this reason, it is conventional practice to precoat the inner surface of the green or uncured tires with a lubricant in order to provide lubricity between the outer bladder surface and inner tire surfaces during the entire molding operation. This lubricant has also been called a bladder lubricant, and is often a silicone polymer dispersed in a solvent or water.
It is to be appreciated that the release of the tire from its cure bladder in an industrial manufacturing setting is intimately associated with both the phenomenon of release (to prevent sticking) and the phenomenon of lubrication (to enhance slipping) between the bladder and the adjacent tire surfaces. The release aspect refers to the basic ability to avoid adhesion, or release and the aspect of lubrication relates to enhancing the ability of the surfaces to slip and enable a movement of the bladder with respect to the tire.
Butyl rubber is commonly used in tire curing bladders. Butyl rubber is a copolymer of predominantly isobutylene with small amounts of diene monomers, usually isoprene to give sufficient unsaturation to allow the butyl rubber to be crosslinked. Copolymers of isobutylene and para-methylstyrene which are subsequently brominated are being manufactured by Exxon. These polymers are useful in many applications where butyl rubber is used.
U.S. Pat. No. 4,863,650 disclosed the use of fillers such as kaolin, chalk, rock dust, silicas, carbon black and graphite in silicone release agent films to result in mat finishes. U.S. Pat. No. 4,310,427 disclosed the use of dry powders such as mica, talc, and graphite which were dusted onto the interior surfaces of "green" tires to provide lubrication and release. U.S. Pat. No. 3,967,978 disclosed the use of fine solid particles such as mica or graphite in the lubricant.
Accordingly, it is desired to provide curing bladders with decreased adhesion to cured tires, increased flex life, and improved heat stability, specifically low, hot tension set.